Composite materials consisting of carbon, glass or polymer fibers embedded in a continuous polymer matrix are well known means of providing strong, lightweight structures. Because of its extremely high strength, carbon is generally the material of choice for the fiber. Typically, the fiber is embedded in a thermosetting polymer, such as epoxy, and cured at elevated temperature and pressure to create the desired structure.
A multi-layer laminated structure of such a composite can be used to obtain different, and sometimes improved, properties such as increased strength and durability. These laminates are typically prepared using either thermoplastic or thermosetting resins. Prepregs can be used in the fabrication of finished articles. A prepreg is fibrous material that has been impregnated with a resin system and partly cured. However, combining prepregs with other materials in a single composite structure has been problematic. For example, thermoplastics and thermosetting resins have different physical properties and require different processing parameters. Thermoplastics are hard at room temperature but become soft upon heating. Thermoplastics generally have little or no cross-linking, and the individual chains of the polymer slip by one another upon heating, resulting in a softening of the material. Thermosetting resins, on the other hand, consist of a polymer that becomes highly cross-linked in three dimensions when cured and assumes a fixed shape in the fully cured condition. In addition to different physical properties, thermosets need relatively long, elevated temperature curing in a fixture that retains the shape of the desired object. Thermoplastics, on the other hand, use a shorter, elevated temperature molding operation that simultaneously forms the object into its final shape.
In order to take advantage of the beneficial properties of both thermoplastic materials and thermosetting prepregs in a single structure, the use of a film of thermoplastic adhesive has been proposed. However, thermoset resins do not chemically adhere to the film with sufficient strength and durability.
Jacaruso, et al. in U.S. Pat. No. 5,304,269, has proposed an alternative method of adhesion of thermoset composite structures. The method disclosed by Jacaruso includes the use of thermoplastic adhesive strips, which are comprised of a layer of semi-crystalline thermoplastic material such as polyetherketone, a layer of amorphous thermoplastic material, a ply of dry fiber reinforcement partially embedded in the layer of thermoplastic material, and a coating of thermoset resin over the exposed dry fiber. Initially, heat is applied to the structure to melt and fuse the semi-crystalline and amorphous thermoplastic adhesive strips by raising the temperature of the two layers above the melting point of the semi-crystalline thermoplastic material. Since the semi-crystalline thermoplastic has a higher melting temperature than amorphous thermoplastic, the two layers are fused together. The composite structure is then cured by conventional methods. This method, however, requires several processing steps and does not allow the formation of complex shapes in a single process step.
Therefore, due to the incompatibility of thermosetting and thermoplastic materials, laminated structures are generally prepared entirely from one or the other. This has hindered the development of laminated structures that combine thermoplastic and thermosetting resins with a strong enough bond that can be molded into complex shapes in a one step process.
The present invention overcomes the previously described limitations of using thermosets and thermoplastics in the same structure. The present invention not only overcomes these limitations but also enables the artisan to construct a composite material with high stiffness, insulating properties and improved x-ray transparency in a single process step.
The present invention provides molded composite materials of substantially uniform thickness and having a core of substantially amorphous thermoplastic and at least one outer layer of thermosetting resin bonded to each surface of the core. The core material and each outer layer are interfacially commingled to provide a bond strength of greater than about 500 psi. These composites can be used to form complex molded shapes of substantially uniform thickness.
The present invention further provides a process for the preparation of a complex molded composite material by assembling in matched molds a core of substantially amorphous thermoplastic and at least one outer layer of thermosetting resin adjacent each surface of the core, and applying heat and pressure to the assembled components sufficient to cure the thermosetting resin and form an interdiffusional bond at the interface between the core and each outer layer.